Receiving apparatus for communication systems



Nov. 16, 1943. w. M. ADAMS RECEIVING APPARATUS FOR COMMUNICATION SYSTEMS Filed March 4, 1942 .RRQ SS si? ...I 4

Patented Nov. 16, 1943 UNITED] STATES PATEN'l1OFFICE:`

nnonrvmo ABrARA'rUs Fon ocirc# i MUNICATION SYSTEMS Warren M; AdamsKokomo,1nd., assignor to The 'y Union Switch and Signa! Company, Swssvale, e Pa., a'corporationof Pennsylvanial applicateur March 4, 1942, serial No. 433,266

f zciaims. (o1. 25o- 20) My invention `relates to receiving apparatus for communication.r systems, and more particularly to receiving apparatus for systems using an oscillating current. 1 f Y Communication systems frequenly require an amplifier of one or more stages, and in systems using an oscillating current such as` a modulated carrier current one or more stages of high frequency amplification are employed ahead of the demodulator which in turn-is followed by one or more stages of low frequency amplification. Suchreceiving apparatus is oftenprovided with automatic volume control to insure a substan tially uniform energy level output. y

A feature of my present invention is the pro-- vision of receiving apparatusfor. communication systems incorporating a-novel and improved elec tron tube amplilier provided with means for controllingthe amplier gainand the volume-of the output energy. y I Y Another feature of my invention is Vthe provision of improved automatic volume control means whereby the grid current of the tube of one stage of a receiving amplifier` is1used to control the ampliiication gain of the tube of one or more stages ahead of such` one stageas 'Well as to control the amplification gain-of such-one stage.

Still another feature of my invention is the provision of improved automatic volume control means whereby distortion of theoutput energy lis minimized.` Other features, objects and advantages of my invention will appear as the specication progresses. l Y

The features,y objects and advantages of'my in vention are attained kby arranging Vthe receiving apparatus to cause the gridA of the tube of a preselected one of the stages to swing positive in po tential with respect to itscathode during atleast a portion ofthe positive half cycles ofthe input energy above a preselected normal value, so that grid current flows. A control unit including a condenser is interposed in this grid 4circuit and the flow of grid current charges the condenser, the stored voltage being proportional to the value of the grid current, andin turn proportional to the magnitude of the Vinput energy. This stored voltage is fed back onto the grid of the tube of one or more'of the earlier stages as abias volt--r age to control the gain at such earlier stages. Furthermore, such stored voltage isv effective to control the bias voltage of the associated tube. I also provide a manually operable biasing means forv readily adjusting the receiving apparatus at the predetermined point at which the automatic control unit is to becomereffective. Lprovidein addition` a manually operable meansA to adjust the magnitude of the voltage applied to the nal stage of the amplifier so that the output volume of the usual loudspeaker can readily be controlled. s

I shall describe one form of apparatus ernbodying my invention, and shall then point out the novel features thereof in claims. y

In the accompanying drawing, Fig. 1 is a diagrammatic View showing one form of apparatus embodying my invention Vwhen used in the re ceiving apparatus for a carrier telephone system. Figs, 2, Sand 4 are diagrams illustrating operating characteristics of the apparatus of Fig. 1.

.Apparatus embodying my invention is peculiarly suitable for use in receiving apparatus for railway train communication systems in which a carrier telephone current is employed, and energy is received on a train inductively from a wayside transmitting circuit which circuit usually includes the track rails. For example, in such a train communicationgsystem, the carrier may be` of the order of 5700 cycles per second and the voice frequencies of 400 to 2500 cycles per second, While a single side -band only is transmit-` ted, whereby the wayside transmitting circuit conveys ra carrier telephone current of the frequency range of 6100 to 8200 cycles.' The attenuation of the current is relatively high due to the rail impedance', ballast leakage and other operating conditions so that the energy picked up by the receiving means mounted on the train in inductive relation to the wayside circuit varies betweenl relatively Wide limits due to the constant change in the position of the train with respect to the sending station and due to the constant change in conditionsof the transmitting circuit. Because the receivedcurrent is at times of a very 10W energy level, a relatively high gain amplifier is required'in the receiving apparatus with the result that when the received current is relatively large vin magnitude the response of the train carried-loud speaker or telephone receiver may be so loud as to be annoying. Furthermore, since the receivingapparatus is normally conditioned for reception, the earth currents and stray magnetic yelds4 may produce noise of a volume that is annoying duringnon-communication periods because of the high gain of the amplier. Al-

' though apparatus embodying my invention thus has peculiarl utilities in such railway train communication systems, my invention isY in no man ner limited in'its use to railway train communication `systems andsuch application is cited as an illustration of the many places where apparatus embodying my invention is useful.

Referring to Fig. 1, the receiving apparatus is shown as including two stages of high frequency amplification, a demodulator and a. single stage of low or audio frequency amplification as well as a receiving unit and a loud speaker. The reference character IC indicates-a receiving unit for picking up energy from a sending station, not shown. In radio systems, the receiving unit IC would be an antenna and in railway train communication systems of the type referred to herein-` before, the receiving unit would be an inductor mounted on a train in inductive relation with the track rails. The first and second stages of amplication include electron tubes 2 and 3, respectively. As here shown, tubes k2 and 3 are of the indirectly heated cathode type, the larnent of each of the tubes being constantly heated in the usual manner. Receiving unit IC is coupled to control grid and cathode 0 of the first stage tube 2 through a transformer TI, the alternating current grid circuit for tube -2 being completed through a biasing unit BUI, ground electrodes 1 and 8, a condenser C2, and a winding of transformer TI. Biasing unit BUI preferably includes an ladjustable resistor in multiple with a condenser 3I. Plate voltage is provided lfor plate 9 of tube'2 from a sourcewhose terminals are indicated at B300 and N300, such plate circuit preferably including terminal B300, resistor I0, impedance Il, another resistor I2, winding I3 of a transformer T2, plate 9 and intervening tube space to cathode 6 of tube 2, biasing unit BUI, ground electrodes 1 and I4 and terminal N300. A condenser I5 is connected across at least a portion of winding I3 of transformer T2 and winding I3 in multiple with condenser I5 are coupled through a condenser I1 to the grid circuit of the second stage tube 3. l

According to my invention,'a control unit VI, comprising a resistor R2 and a condenser CI in multiple, is interposed in the grid circuit of the second stage tube 3, such grid circuit including control grid I6v of tube 3, a resistor RI, control unit VI, ground electrodes I9 and 1, biasing unit BUI and cathode I8 of tube 3. Furthermore, the

control unit VI is connected to the grid circuit of therst stage tube 2 through aconnection including resistor`R3 connected between the terminal of unit VI toward control grid I0 of tube 3 and the terminal of the winding of transformer TI interposed in the grid circuit of tube 2. The function of control unit VI and this connection including resistor R3 will be explained more fully hereinafter.

The plate circuit for the second stage tube 3 extends from terminal B300 through resistor I0, impedance II, resistorIZ, winding 20 ofa transformer T3, plate 2l, intervening tube space lto cathode I8, biasing unit BUI and'ground electrodes 1 and I4 to terminal N300. It is to be seen that each tube2 and 3 is provided with a normal grid bias voltage derived from unit BUI, the magnitude of such bias voltage being predetermined by the setting of the resistor 30. As here shown, tubes 2 and 3 are provided with screen grids 22 and 23, respectively, and each of which screen grids is connected to the cathode of the associated tube. Also, other screen grids 24 and 25 of tubes 2 and 3, respectively, are supplied with a positive voltage from the E300-N300 current source in the usual manner as will be apparent from anA inspection of Fig. 1.

v The plate circuit of tube 3'is coupled to the input side of a demodulator DO by transformer T3, Whose winding 20 is included in the plate circuit of tube 3 and whose windings 26 and 21 are connected to demodulator DO. The specific construction of demodulator DO is immaterial and it is shown conventionallyfor the sake of simplicity. For example, demodulator DO may be of the balanced type covered by Letters Patent of the United States No. 2,129,313, granted September 6, 1938, to L. D. Whitelock, for Apparatus for electric signaling systems. It is suiiicient for this application to point out that in demodulator DO a carrier is mixed with the side band telephone current appearing in windings 26 and 21 of transformer T3 and the voice frequencies appear in winding 28 of a transformer T4, the carrierand the other products of demodulation being eliminated at the demodulator and at a band pass filter BPF.

The voice frequencies appearing in the output of demodulator DO are further amplified at lan audio stage which includes an electron tube 4, preferably of the indirectly heated cathode type, the filament of tube 4 being constantly heated in the usual manner. A grid circuit is connected across control grid 32 and cathode 33 of tube 4 to include a biasing unit BU2 and ground electrodes 34 and 35, and to be coupled to Winding 28 of transformer T4 through a resistor RL having an adjustable contact 40. Plate 36 of tube 44 is supplied with voltage from the B300-N300cur rent source through a winding 31 of a transformer T5 having a winding 38 connected to a loud speaker LS. Consequently, the voice frequencies are amplified at tube 4 and reproducedat loud speaked LS, the voltage applied to tube 4 being predetermined by the setting of the adjustable contact 40 of resistor RL.

The parts of the receiving apparatus for the rst and second stages are so proportioned that when input energy above a predetermined magnitude is picked up by the receiving unit IC and amplified at the first stage tube 2, the grid I6 of the second stage tube'3 is caused to swing positive in potential with respect toy cathode I8 during at least a portion of each positive half cycle of the received current. During the period grid I6 swings'positive in potential, it tends to draw grid current, such current flowing in resistor R2 of unit VI to build up a charge on condenser CI of a value substantially equal to the average voltage drop across resistor R2, and 'hence proportional to the magnitude of the grid current and in turn proportional to the magnitude of the input energy. The polarity of the charge thus created on condenser C l is as indicated by the plus and minus signs placed in Figjl. That is to say," the terminal of control unit VI adjacent the ground electrode I9 is positive and the terminal toward control grid I6 of tube 3 and to which terminal the connectionl including R3 is connected is negative. It is clear that the charge built 'up on condenser CI of 'control unit VI is applied to the control grid 5 of the rst stage tube 2 through the connection including resistor R3 and is also applied to the control grid vII of the-asso-rv ciated tube 3, and the polarity of this charge-is such as to increase the normal negative grid bias voltage of these tubes `derived from' the biasing' unit BUI, with the result that both tubes 2 and 3 amplify at a less extent whenv the incoming'em ergy is of a magnitude above the predeterminedy value. The energy level ofthe current applied to demodulator DO is therefore made of substan-l tially the'sarne value for-input energy of large magnitude as for input energy of a normalpredetermined magnitude. After demodulation, the voltage drop across resistor RL is substantially uniform, and hence the voltage applied to control grid 32 of the iinal stage tube 4 is predetermined by the setting of contact 40 of resistor RL. Consequently, the output volume of the loud speaker LS can be governed by manually adjusting the position of contact 40 along resistor RL. Y

In'Fig. 2, curves plotted between volts input and current output are illustrated. Curve B is that attained by the receiving apparatus through automatic control unit VI and its associated connection as shown in Fig. 1. It is to be noted that an output of about 250 milliamperes is quickly reached as the input voltage reaches a normal predetermined value of about .001 volt and the output current remains at substantially the same level for input voltages of larger magnitude. The dotted line curves BI and B2 are curves resulting When the manually controlled resistor 30 of biasing unit BUI is set to effect larger normal bias 1 voltages for tubes 2 and 3. The curve A illustrates the slope of the volume control obtained when a receiving ampliiier is provided with the usual form of automatic volume control means including a rectier in the output of the low frequencystage to rectify a portion'of the output current to produce a voltage which is applied to the grid of the earlier stages in the usual manner.

I have found by test that there is little distor- 'tion introduced at the earlier stages by the automatic volume control of my invention. In cases of a relatively low or normal value of input voltage (see Fig. 3) the normal grid bias voltage (EG) is such that the resultant amplied current (IP) is without distortion as illustrated in Fig. 3. At higher values of input voltage the volume control acts to shift the bias voltage almost immediately so that, as illustrated in Fig. 4, the bias voltage on the rst tube is shifted from EGI to EG2 and the amplified current therefrom is still Without distortion. l

It is to be understood that the receiving apparatus of Fig. 1 can include additional stages of high frequency amplification andV additional stages of low frequency amplification if desired.

Receiving apparatus here disclosed has the advantages that the normal bias voltage can readily be predetermined by means of adjustable unit BUI to bring out the voice current over noise energy that may be present. Resistor RL can be adjusted to apply a voltage to the final stage to cause a .desired volume of the loud speaker output as required to be heard above local noise, as might be the case in the cab of a railway locomotive. Also the automatic volume control effected by unit VI risesto a high value quickly system using an oscillating current, the combination'comprising; a first and a second stage 0f amplification each of which stages includes an electron tube having a plate, a cathode and a control grid; a grid circuit for said iirst stageV tube and including a Winding disposed to receive such oscillating current and a biasing unit hav.- ing an adjustable resistor, a grid circuitfor said secondstage tube including said biasing unit and a control unit having a resistor and Aa condenser in multiple, a plate circuit for each of said tubes and each of which plate circuits includes a source of direct current and said biasing unit, means to couple the first stage tube plate circuit to the second stage tube grid circuit, said biasing unit having one terminal connected tothe cathodes in multiple ofthe first and second stage electron tubes to provide a normal negative grid bias voltage for each of said tubes due solely to the plate currents flowing in such unit and which voltage is of a value predetermined by adjustment of said adjustable resistor, said control unit having one terminal connected to the control grid of the second stage tube and its other terminal connected to the cathode of the second stage tube through said biasing unit to create a charge across the condenser of the control unit When the oscilon input voltages over a predetermined value and the output energy level is held at a substantially uniform level. Furthermore, the automatic volume control causes little distortion of the outputV energy.

Although I have herein shown and described but one form of receiving apparatus for communication systems embodying my invention, it is,

understood that various changes andV modifications may be made therein within the scope of the appended claims Without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In receiving apparatus for a communication lating current received by Y'said Winding and amplied by said first stage tube drives the 'second stage tube control grid positive in potential with respect to the cathode and grid current flows, and means including another resistor to connect said control unit to the grid of said iirst stage tube to increase its negative grid bias voltage by the charge of said condenser.

2. In receiving apparatus for a communication system using a carrier telephone current, the combination comprising; a first and a second stage of high frequency ampliiication, a demodulator and a stage of audiofrequency amplification; each of said stages including an electron tube having a plate, cathode an da control grid; a first grid circuit including a receiving unit adapted to pick up such telephone current and connected to the grid and cathode of said first stage tube, a second grid circuit including a control unit comprising a resistor and acondenser in multiple and connected to the grid and cathode of said second'stage tube, aplate circuit for each ,of said first and second stage tubes and including Vthe first stage tube when telephone current of large magnitude is picked upV and grid current flows in the grid circuit of the second stage tube to charge the condenser of said control unit, means to couple the plate circuit of said second stage tube to said'demodulator, a third grid circuit including awindng and'an adjustable resistor in multiple connected to the grid and cathode of said audio stage tube, means to couple said y winding to said demodulator to apply a voltage to said audio stage tube as, predetermined by said adjustable resistor, and aloud speaker coupled f to the plate circuit of said audio stage tube.

WARREN M. ADAMS. 

